Just started Prism's Waterfall Vest using the cartridge belt stitch pattern. The pattern itself is simple, but the instructions given say to increase one stitch on each end every other row and work them into the pattern as you go.

These increases are really throwing me for a loop. Has anyone knit this pattern? Recommendation for using kfb or m1 for the increases? Ordinarily I'd make the increase in the secod stitch but at the end of the row one the last two stitches are sl1 k1 and I don't think I should increase in the sl1.

I routinely use KRL or KLL (see videos for Increases) but it's really your preference here. Either kfb or m1 will work. I agree about working the increases one st in from the edge and since the first st is a slip st, you could even work 2sts in.

There may be some kind of border or background st to the pattern, stockinette or rev stocknette for example. Do the increases in that stitch until you have enough for a repeat (or even a partial pattern repeat). You can do the increase in one row and then add the increased stitch into the pattern in subsequent rows. Just try for some consistency in how you do this and it will look fine.